1. Field of the Invention
The present invention relates to a solid state imaging device having a microlens array formed on a pixel array thereof.
2. Description of Related Art
A variety of solid state imaging devices each of which comprises a light receiving element having an MOS-type light receiving/accumulating portion, are well-known in the prior art. Among these there is a solid state imaging device of a type which uses a light receiving element having the MOS-type light receiving/accumulating portion, as well as having an internal amplifying function. The solid state imaging device of this type is exemplified by an imaging device which uses a CMD (Charge Modulation Device) suggested by the inventor of the present invention. The contents of the above-described imaging device were disclosed in Japanese Patent Laid-Open No. 61-84059 and a thesis entitled "A new MOS Image Sensor Operating in a Non-Destructive Readout Mode", p.p. 353 to 356, collected preliminary theses of International Electron Device Meeting (IEDM) held in 1986.
FIG. 1 illustrates a cross sectional structure of one pixel of the conventional CMD solid state imaging device. Referring to FIG. 1, reference numeral 101 represents a p.sup.- Si substrate, 102 represents a channel layer composed of an n.sup.- epitaxial layer, 103 represents n.sup.+ source (drain) layer, 104 represents n.sup.+ drain (source) layer, 105 represents an SiO.sub.2 gate insulating film, 106 represents an annular gate electrode and 107 represents a protection film (passivation film) made of insulating material.
The light receiving operation performed by the CMD light receiving device structured as described above will now be described. When light 108 is made incident upon the upper portion of the gate electrode 106, incidental light 108 passes through the protection film 107, the gate electrode 106 and the gate insulating film 105 before it is introduced into the channel layer 102. As a result, hole-electron pairs are generated in the channel layer 102. Light generating holes of the above-described hole-electron pairs are accumulated in the interface between the gate insulating film 105 formed below the gate electrode 106 and the channel layer 102, the gate electrode 106 being applied with an inverse bias. As a result, the surface potential of the interface is raised. Thus, the potential barrier acting against the electrons present between the source layer 103 and the drain layer 104 is lowered. Thus, an electron current passes through the n.sup.- channel layer 102. By reading the above-described electron current, an optical signal, which has been amplified, can be obtained.
There is a new technology relating to the imaging device such as a CCD, which has been put into practical use, in which the aperture factor is improved by means of a microlens array accumulated on the imaging device by using a resin. For example, Japanese Patent Laid-Open No. 1-309370 discloses such a structure arranged as shown in FIG. 2. That is, the solid state imaging device disclosed above includes a light receiving portion comprising n.sup.+ photodiode regions 202 forming a multiplicity of photoelectrical conversion elements formed on a p-type Si substrate 201, n-type buried channels 203, p.sup.+ type channel stoppers 204, transfer electrodes 205, via an interlayer film 206, disposed at the positions except for the positions which correspond to the n.sup.+ photodiode regions 202 and aluminum light insulating films 207 disposed on the interlayer film 206 at positions which correspond to the transfer electrodes 205. The solid state imaging device further includes transparent intermediate layers 208-1, 208-2, 208-3 and 208-4 which cover the light receiving portion and each of which has a flat surface, and a convex lens comprising transparent photosensitive resin layers 213 selectively formed on the above-described intermediate layers 208-1, 208-2, 208-3 and 208-4 at positions to correspond to the n.sup.+ photodiode regions 202 and another transparent intermediate layer 208-5 for covering the surfaces of the transparent photosensitive resin layers 213. The above-described intermediate layers 208-1 to 208-5 are made of PGMA, while each of the transparent photosensitive resin layers 213 is made of gelatin. Referring to FIG. 2, reference numerals 209, 210 and 211 respectively represent magenta, cyan and yellow coloring layers formed on the intermediate layers 208-1, 208-2 and 208-3.
In the imaging device thus-constituted, each of the intermediate layers 208-1 to 208-5 and the transparent photosensitive resin layers 213 has a refractive index of 1.5 which is substantially the same as that of each of the coloring layers 209, 210 and 211. Assuming that light is made perpendicularly incident upon the convex lens and, assuming that the thickness of the intermediate layer is t.sub.1 and that of the convex lens is t.sub.2 as shown in FIG. 3, an aperture factor which approximates to 100% is realized if the following equation is satisfied: EQU t.sub.1 =n.sub.1 /(n.sub.1 -n.sub.0).multidot.(p.sub.2 +t.sub.2.sup.2)/2t.sub.2 -t.sub.2
where n.sub.0 and n.sub.1 respectively are the refractive index of air and that of the intermediate layer and symbol p is the half of the pitch of cells in the horizontal direction.
That is, by forming a microlens having a thickness of t1 on the light receiving portion, according to the above-described equation, with respect to the pitch p, incidental beams can be diaphragmed onto substantially one point on the surface of the light receiving portion. Furthermore, in a structure in which the focal point is positioned in the light receiving region, a high aperture factor of about 80% can be realized.
The sensitivity can, to a certain degree, be improved in comparison to the conventional solid state imaging device by arranging the structure in such a manner that the amplifying type light receiving element is used in the pixel portion of the solid state imaging device and the microlens is disposed on the light receiving portion. However, a solid state imaging device capable of realizing further improved sensitivity is also desired.